(*transaction).FetchBlockHeaders

Description

FetchBlockHeaders returns the raw serialized bytes for the block headers
identified by the given hashes. The raw bytes are in the format returned by
Serialize on a wire.BlockHeader.

Returns the following errors as required by the interface contract:

- ErrBlockNotFound if the any of the requested block hashes do not exist
- ErrTxClosed if the transaction has already been closed
- ErrCorruption if the database has somehow become corrupted

NOTE: The data returned by this function is only valid during a database
transaction. Attempting to access it after a transaction has ended results
in undefined behavior. This constraint prevents additional data copies and
allows support for memory-mapped database implementations.